1. Field
This invention relates to solid state devices utilizing ballistic electron behavior. It is specifically directed to solid state nonthermionic cathodes.
2. State of the Art
Cathodes are conventionally thermionic in nature. Thermionic cathodes are widely used, and have been the only practical devices available for most applications requiring an electron beam. The design of practical electronic devices has thus been constrained by the power requirements and other limitations inherent in thermionic cathodes.
A great need exists for nonthermionic cathodes with long lifetimes, high current densities, and with the ability for instant "turn-on." A practical solid state cathode could replace the thermionic cathodes in conventional cathode-ray tubes. It could also serve as an electron source for flat-panel displays; as a source of electrons for microwave traveling wave tubes, gyrotrons and free-electron lasers; and as an electron source for vacuum microelectronics.
Current lithographic and crystal growth technologies are capable of constructing compound semiconductors with layers having thicknesses less than the mean free path of electrons in the material. Over distances of this magnitude, an electron can be accelerated to energies not limited by collisions in the material. Electrons behaving in this manner are termed "ballistic electrons." The effects of lattice vibrations (phonon scattering) within a lattice are reduced when electrons travel ballistically.
A barrier to the development of a practical solid state cathode has been the difficulty in overcoming the work function of the emitting material. Though devices based upon field emission or avalanche emission have met with varying degrees of success, they suffer from inherent drawbacks such as short lifetimes, low emission, and low efficiency.
Recently, the phenomenon of ballistic transport in semiconductors has become recognized for its potential in high-speed electronics. The ability to fabricate thin (10-100 nanometer (nm) layers has made it possible for an electron to travel the length of a layer without experiencing the effects of scattering within the lattice, i.e., "ballistically." Under this condition, an electron acts much like a particle in free space with a reduced mass, and it can be accelerated to considerable energies. At high energies, the electron interaction with the periodic lattice potential becomes stronger, and the velocity peaks to a material dependent value (about 10.sup.8 centimeters per second (cm/s) in GaAs). This peak velocity is a design objective for ballistic transistors because it results in the shortest transit times through each semiconductor layer; it corresponds to an electron energy of about 0.36 electron volts (e.V.) in gallium arsenide (GaAs) for example.
This disclosure will use terms in the context and sense generally recognized by those skilled in the art of solid state devices. For clarity, unless some other meaning is clear from the context in which a term is used, the following definitions are adopted for purposes of this disclosure.
Accelerate: To increase the energy of an electron by means of an electric potential. PA1 Active Length: The distance over which an electron travels ballistically. PA1 Ballistic Electrons: Electrons whose travel within a material are not limited by collisions within the material. PA1 Ballistic Injector: A device by which ballistic electrons are introduced into a material. PA1 Ballistic Material: A solid state element or compound in which electrons can be caused to behave ballistically. The currently preferred ballistic materials are semiconductors, but metals and insulators may also be capable of this function and may be considered as ballistic materials in specific instances. PA1 Band Gap: The minimum energy required for a valence electron in a semiconductor to become a conduction electron that can move more freely throughout the material. PA1 Compound Semiconductor: a semiconductor made of a compound of two or more elements (instead of a single element like silicon). For example, to make III-V semiconductors, one or more elements having three valence electrons, such as aluminum, gallium, and indium (those in Group III of the periodic table) are combined with one or more elements having five valence electrons, such as phosphorus, arsenic, and antimony (those in Group V). PA1 Depleted Region: A region in a material that is depleted of charge carriers. PA1 Doping: The introduction of a secondary material to the primary material of a semiconductor to provide an excess of charge carriers (either holes or electrons) in the primary material. PA1 Emitter: The stage of a cathode device which allows electrons to exit from the cathode. PA1 Heterojunction: An interface formed by adjacent layers of materials having different chemical compositions. PA1 Injected Electrons: Electrons which are introduced into the environment, a vacuum or a material from an injector. PA1 Injector: A source device of injected electrons. PA1 Intrinsic: An undoped semiconductor material. PA1 Layer Length: The linear distance through a material layer in the direction of current flow. PA1 Majority Carrier: An electron in an n-type doped material or a hole in a p-type doped material. PA1 Minority Carrier: An electron in a p-type doped material or a hole in an n-type doped material. PA1 Mean free path: The average distance an electron can travel between collisions in a material. PA1 Phonon: A quantum of thermal vibration that propagates through a material. PA1 Semiconductor: A material with electrical properties which are intermediate the electrical properties of metals and insulators, respectively. PA1 Tunneling: The quantum mechanical property of electrons which allows an electron to overcome an energy barrier at an energy below the barrier energy. PA1 Work Function: The energy required to remove an electron from a material into a vacuum.
References to "before," "following," or similar terms are with respect to the direction of electron flow in a device.
It is known that an electron can travel ballistically through a material over distances on the order of the mean free path in the material. Over this distance, ballistic electrons can attain kinetic energies far greater than can electrons traveling non-ballistically.
Ballistic behavior of electrons and proposed devices utilizing ballistic electron behavior are discussed in the article "Ballistic Electrons in Semiconductors," Heiblum and Eastman, Scientific American, February 1987, pp. 102-111. This article is incorporated by reference in this disclosure for its general explanation of ballistic behavior and devices, particularly its explanation of heterojunctions and other types of ballistic electron injectors. Two types of ballistic injectors are described: planar-doped barrier and tunnel barrier injectors.
In a typical planar-doped injector, a thin layer of beryllium atoms represents a potential hill over which electrons cannot climb. Applying a bias potential across the outside semiconductor layers raises the potential on one side of the hill, allowing electrons to cross the beryllium layer and accelerate down the other side of the hill. The electrons exit ballistically. An advantage of the planar-doped injector is that the velocity of the injected electrons can be controlled by the applied bias.
Structurally, the tunnel injector sandwiches an undoped semiconductor layer between outer layers of doped semiconductor. It depends upon the energy differences between different layers to accelerate electrons. A typical tunnel injector has a thin aluminum gallium arsenide layer sandwiched between doped gallium arsenide surfaces. A bias potential applied across the outside layers allows the electrons to tunnel through the thin intermediate region. When the size of the barrier matches a quarter wavelength of the tunneling electrons, a current peak is achieved. An advantage of the tunnel structure is that by biasing the structure to either side of the current peak, the injected electron current can be modulated.
Although both the planar-doped and tunnel injection devices produce a source of ballistic electrons, they cannot function as solid state cathodes because all of the injected electrons are gathered by a collector.
U.S. Pat. No. 4,352,117 discloses a negative electron affinity device constructed in solid state material by providing a semiconductor body with an electron barrier over most of its surface. A small opening in the barrier exposes the semiconductor material. A negative electron affinity material with a work function lower than the energy of excited electrons in the semiconductor material is placed in contact with the semiconductor material in this opening. Electrons migrate non-ballistically to this opening, and are injected into the environment through the lower work function material.